Peter MacNicol was not related to Jimmy and Kristy McNichol -- notice the name is spelled differently -- but everyone thought he was. In fact, everyone at Augustana College went to Dragonslayer in 1981 because they mistakenly believed that Peter was the buffed 21-year old teen idol.
It wasn't good. Derivative, heterosexist...and sword-and-sorcery heroes are supposed to be man-mountains, like Arnold Schwarzenegger. Or Jimmy McNichol. But Peter was scrawny!
But we get to see Peter's scrawny physique, and there's a gay subtext with the always flamboyant Kevin Kline.
Remember in Ghostbusters (1984), Rich Moranis plays a nerd with a crush on Dana, who becomes possessed by the evil spirit? Ghostbusters II (1989), which nobody saw, had precisely the same plot, with Peter as the nerd with a crush on Dana who's possessed by the evil spirit.
You probably saw him as the nerdy villain Gary Granger, summer camp manager who tried to force Wednesday and Pugsley into conformity in Addams Family Values (1993).
And as Renfield, snively servant of the wisecracking vampire in Mel Brooks' parody Dracula: Dead and Loving It (1995).
He got into a bit of a controversery in 2001, when John romances a woman played by Anne Heche, who had just announced that she had "become" a lesbian. Would audiences accept a hetero-romance played by a lesbian?
Apparently it wasn't a problem, and later Heche "turned back" to straight.
Since Allie McBeal, Peter has starred in Numb3rs, 24, and Grey's Anatomy, and done a lot of voice work, notably playing X the Eliminator, gay-vague fanboy and wannabe arch-nemesis of Harvey Birdman on Adult Swim.
See also: Jimmy McNichol and the Gay Coach